1. Field of This Invention
This invention relates to the production of maleic anhydride from the process gases from the catalytic oxidation of a hydrocarbon in the steam phase.
2. Prior Art
Maleic anhydride has been produced by the catalytic oxidation in the steam phase of benzol (i.e., benzene), and occasionally also n-butane, butene and other C.sub.4 -fractions. According to known methods, the maleic anhydride is obtained from the process gases in the following manner.
First, the gases are cooled to a temperature of 55.degree. to 60.degree. C. Such temperature lies below the so-called thaw point of maleic anhydride, but still is above the thaw point of water, which for its part is a by-product of the reaction. In the benzene process approximately 50 to 60 percent of the maleic anhydride can be condensed from the process gases. The lower the condensation temperature which is chosen, the more the water contained in the gas will be bound by the condensing maleic anhydride, thus forming maleic acid. However, the formation of maleic acid is undesirable, since the maleic acid is a solid product and is poorly soluble in molten maleic anhydride. Such leads to plugging up of the apparatus, which, in turn, results in the loss of throughput and yield.
The uncondensated maleic anhydride is obtained in the form of maleic acid by washing the stream of gas with water. The maleic acid then has to be converted again by dehydration into the maleic anhydride. As a result of isomerization, fumaric acid may then also be formed which ultimately leads to a large loss of yield.
U.S. Pat. No. 3,891,680 describes carrying out the washing operation in esters formed from phthalic acid with C.sub.4 -C.sub.8 alcohols. According to German O.S. 2,444,824, dibenzylbenzol is used as the wash fluid. Finally, British Pat. No. 1,443,411 describes polymethylbenzophenone as the wash fluid. However, such systems have grave defects, such as the washing fluid lacking stability, especially (i) in the case of phthalic acid ester which leads to additional expenditures of the replacement of the lost washing fluid, (ii) from the high cost for the washing fluid in the case of polymethylbenzophenone, and (iii) from the fact that both polymethylbenzophenone as well as dibenzylbenzol are solid at ambient temperature.